1. Field of Invention
This invention relates to color television signal transmission, and more particularly to a method and apparatus for encoding a television signal to reduce apparent dot-crawl while maintaining compatibility with the standards of the National Television Systems Committee (NTSC).
2. Description of Related Art
The term "dot-crawl" refers to a phenomenon that occurs at a sharp vertical transition between contrasting colors in an NTSC encoded color television signal. The sharp vertical transition is rendered with a jagged edge which resembles a zipper having teeth which appear to progress (i.e. crawl) up the screen. This noticeable and undesirable effect results from the method in which color information is encoded in an NTSC color television signal.
The NTSC color system is a type of color transmission approved by the Federal Communications Commission in the United States to be compatible with pre-existing monochrome receivers. The approach used is to transmit encoded color information on a subcarrier 3.579545 MHz from the main picture carrier. The main picture carrier is modulated with a luminance signal Y which is derived from red (R), green (G), and blue (B) camera signals according to the following equation: EQU Y=0.299R+0.587G+0.114B.
The color subcarrier is modulated with in-phase (I) and quadrature-phase (Q) signals derived from the color difference signals (R-Y) and (B-Y) according to the following equations: EQU I=0.74(R-Y)-0.27(B-Y) EQU Q=0.48(R-Y)+0.41(B-Y).
Alternatively, the I and Q signals may be directly derived from the R, G, B signals. In any case, the R, G, B signals may be recovered at the receiver from the Y, I, and Q signals.
The signals are used to create a television picture at a rate of 29.97 frames per second, with 525 lines per frame. Consequently, the signals have strong harmonic components that are spaced 15,734.264 Hz apart. To minimize interference that a monochrome receiver might experience from the signals on the color subcarrier, the subcarrier frequency was chosen to be an odd-half-multiple of 15,734.264 to cause the harmonic components of the color signals to interleave with the harmonic components of the luminance signal. The precise color subcarrier frequency is 227.5 times 15,734.264 Hz. This provides a wide frequency separation, frequency interleaving, and minimal interference with the audio signal, which is modulated on an audio subcarrier 4.5 MHz above the main picture carrier.
Since the color difference signals are communicated on a subcarrier having 227.5 cycles per line, and since the television picture frame is composed of two interleaved fields of 262.5 scan lines each, the subcarrier goes through 59,491.25 cycles during the picture scan from one line to the adjacent scan line above it, through 119,437.5 cycles during a complete scan from one line back to itself, and through 178928.75 cycles to the adjacent scan line above it again. Hence during the display of four picture fields, the phase of the subcarrier at a point on the screen progresses through a phase angle of 0, 90.degree., 180.degree., 270.degree., and repeats. This characteristic becomes noticeable near a sharp vertical transition between colors, which broadens the spectra of the color difference signals enough to cause interference with the luminance signal. The luminance signal is displayed having interference near sharp color transitions, with the phase of the interference cycling through 90.degree. of phase for each field displayed. This manifests as a fine pattern of dots that apparently traverse upward along the transition at a rate of one scan line per field, about eight seconds through the height of the image.
A related phenomenon termed "hanging dots" occurs when a sharp horizontal transition between saturated colors occurs, which also broadens the color difference spectra enough to cause interference with the luminance signal. Once again, the luminance signal is displayed having interference with the phase of the interference cycling through 90.degree. of phase for each field displayed. This time, a horizontal line of moving dots is displayed at the transition.
The sharp, contrasting color transitions are common in computer generated graphics, and are becoming more commonplace with the increase in use of computer generated graphics for television display. Dot-crawl and hanging dots are objectionable because their moving nature tends to distract the viewer from the object being displayed. It is desirable to stop the apparent motion of dot-crawl and hanging dots.
Previous patents have provided three methods for "freezing" dot-crawl by altering the signal encoding. In U.S. Pat. No. 4,660,074 issued to Jonathon M. Schine on Apr. 21, 1987, a clock divider circuit in the encoder is used to generate the horizontal synchronization signal, and once for each scan line (except the last scan line of each field) an early reset is issued to the clock divider to cause the synchronization timing of the system to drop a half of a color subcarrier cycle at the end of each scan line. In U.S. Pat. No. 4,661,840 issued to Arthur C. Phelps on Apr. 28, 1987, a clock divider circuit in the encoder is used to generate the horizontal synchronization signal, and once for each field an early reset is issued to the clock divider to cause the synchronization timing of the system to drop a quarter of a color subcarrier cycle at the end of each field. In U.S. Pat. No. 4,833,523 issued to John T. Lentz on May 23, 1989, a clock divider circuit in the encoder is used to generate the horizontal synchronization signal, and at the beginning of each frame the phase of the horizontal synchronization signal is alternately shifted forward and backward by one half of a color subcarrier cycle.
In each of these patents, the synchronization signals are shifted by a fraction of a color subcarrier cycle sufficient to cause the phase of the color subcarrier (and hence any color signal interference) to remain the same from frame to frame. This "freezes" the effective dot-crawl and hanging dot motion. The alterations of synchronization signal timing produce signals that are not truly NTSC compliant, and which may be unsuitable for some NTSC monitors and VCRs, particularly those that use comb filters to separate the luminance signal from the color signals.
One other method which has been used is to invert the color subcarrier for alternate frames. This sudden change of color subcarrier phase leads to visual artifacts such as false colors and color smearing at the top of the display, making this solution unsatisfactory as well. It is desirable to have a method for producing an NTSC compliant signal without dot-crawl and hanging dot motion to improve the display of computer generated graphics on television monitors.